The present invention relates to semiconductor wafers and more particularly to a unique multilayer epitaxial structure having heavily doped layers and to a method for fabricating same by epitaxial techniques in a single continuous operation in the chamber of a chemical vapor deposition reactor.
Semiconductors of many types are formed on wafers which are commonly made of silicon crystal material. The material is produced by an epitaxial grown process in accordance with well known chemical vapor deposition principles. The growth of the material takes place in vessels called chemical vapor deposition (CVD) reactors specially designed for this purpose. Such reactors have facility to control growth conditions such as temperature and pressure, as well as impurity concentrations.
Certain types of semiconductors such as power diodes and Zener diodes have specific electrical parameter requirements, for example, forward voltage drop, leakage current and breakdown voltages, which are difficult to accurately control by conventional diffusion means. A multilayer epitaxial structure as proposed in this disclosure will provide accurate controls to meet the device electrical parameters requirements. In addition, the said proposal will eliminate the need for impurity diffusion.
The said multilayer structure is epitaxially grown with a top layer having high impurity concentration and middle layer having light impurity concentration deposited on a substrate of high impurity concentration with opposite or same dopant type.
However, the formation of heavily doped layers by epitaxial growth usually means slow growth rates and the presence of high impurity concentrations in the material leads to problems with "outdiffusion", where the high dopant concentration layers tend to contaminate low concentration layers, as the material containing the heavily doped layer is heated to high temperature for processing. Another problem associated with heavily doped epitaxially grown layer is "autodoping" in which dopant moves into the gas stream. This autodopant together with highly concentrated dopant gas used, contaminates epi reactor chamber and quartz ware and therefore tends to degrade resistivity control on subsequent runs (the so called "memory effect"). Consequently, where high impurity concentrations are required, it has been necessary to form the doped layer by diffusion in a separate vessel called a diffusion tube.